The present invention relates to an electronic circuit equipped with a radio communication function. More specifically, the invention relates to a small and low-power-consumption electronic circuit effectively applied to construct a sensor network system.
In recent years, there has been considered a network system (hereinafter abbreviated as “sensor net”) for capturing in real time various real-world information into an information-processing device by adding, to a sensor, a small electronic circuit having a radio communication function. For the sensor net, widespread application is considered. For example, there is also considered the application to a medical technology for: always monitoring the pulse etc. through a small finger-ring type electronic circuit in which a radio circuit, a processor, a sensor, and a battery are integrated; transmitting a result of the monitoring to a diagnostic unit by radio communication; and determining a health condition based on the result of the monitoring (see Non-Patent Document 1: Sokwoo Rhee et al, “Artifact-Resistant Power-Efficient Design of Finger-Ring Plethysmographic Sensors”, IEEE Transactions On Biomedical Engineering, Vol. 48, No. 7, July 2001, pp. 795-805).
However, to bring the sensor net into even wider practical use, it is essential that an electronic circuit having power supply of a radio communication function, a sensor, and a battery, etc. (hereinafter abbreviated as “sensor node”) is in a maintenance-free state for a long time and sensor data is being transmitted and the external dimension thereof is reduced. For this reason, the development is in progress to provide an ultra-miniature sensor node that can be mounted anywhere. At the present stage, it has been contemplated that the sensor node, which is practically available for about one year without exchanging its battery, is required from the viewpoint of both of its maintenance and usability.
As described above, the sensor node is required to be very small and, concurrently, to be operable with low power consumption. For example, Non-Patent Document 2 (Crossbow, “Smarter Sensors In Silicon” (online) (Retrieved on Feb. 16, 2004), Internet (URL: http://www.xbow.com/Support/Support pdf files/Motetraining/Hardware.pdf) describes a prototype of a small sensor node called “Mica2Dot” having a diameter of about 3 cm. The Mica2Dot is configured of an RF chip having an integrated functionality necessary for radio communication and a low-power-consumption processor chip. This prototype performs an intermittent operation in such a manner that the prototype is held in a standby mode for 99% of operation time, and is intermittently activated only for remaining 1% of the operation time and wirelessly communicates the result, whereby the operation can be performed for about one year with a small battery. This sensor node performs the radio communication by using, for example, frequency bands ranged from 260 to 470 MHz and 902 to 928 MHz, etc. which are usable without a license in the United States. In general, to perform radio communication, a specific transmitter/receiver device preliminarily licensed or authorized must be used, so that use is greatly restricted. Under such circumstances, since a license-free frequency is used, the device can be mounted without using much labor and cost and the system construction becomes easy, so that it is advantageous to use the above-mentioned frequency bands. Particularly, in the United States, the maximum values of transmission power are regulated as being 11 mV/m or lower in a band of 433 MHz and 50 mV/m or lower in 900 MHz (at a distance of 3 m from a transmission point in any of the cases), wherein even in the license-free frequency bands, when the above-mentioned frequency bands for which the limit values are relatively reduced are used, excellent communication performance and low costs are implemented.
Nevertheless, however, radio wave regulations differ depending on the nation, location, and the like. For example, in Japan, the radio wave regulations are described in Non-Patent Document 3 (“Popularly Used Micro-force Radio Devices” (online) (Retrieved on Feb. 17, 2004), Internet (URL: http://www.circuitdesign.jp/jp/technical/technical_pdf/bijaku.pdf.PDF), and tolerances of electrical field intensity for use in the license-free frequency bands are not necessarily alleviated.
Also, Non-Patent Document 4 (“Data Sheet CR2032” (online) (Retrieved on Feb. 21, 2004), Internet (URL: http://www.maxell.co.jp/e/products/industrial/battery/pdf/CR2032_Data Sheet.pdf) discloses a small button battery suitably usable as power source of the sensor node.